Method and apparatus for handling out-of-order packets during handover in a wireless communication system

Abstract

Techniques for sending packets and maintaining synchronization during handover is described. A user equipment (UE) may be handed over from a source base station to a target base station. The source base station may forward packets for the UE to the target base station, which may receive the packets out of order. In one design, the target base station may determine whether each packet can be sent in order to the UE, send the packet if it can be sent in order, and discard the packet otherwise. In another design, the target base station may re-order packets received within a re-ordering window and may send the re-ordered packets to the UE. In yet another design, the target base station may process each packet received out of order as if the packet is in order, e.g., by incrementing a hyper-frame number (HFN) or re-assigning the packet with a later sequence number.

What is claimed is: 
     
       1. A method for wireless communication, comprising:
 receiving a first packet with a first sequence number; 
 processing the first packet for transmission to a receiving entity; 
 receiving a second packet with a second sequence number earlier than the first sequence number, the second packet being received out of order with respect to the first packet; 
 re-assigning the second packet with a third sequence number later than the first sequence number; and 
 processing the second packet for transmission to the receiving entity, the second packet being processed as if the second packet is later than the first packet. 
 
     
     
       2. The method of  claim 1 , wherein the receiving entity is a user equipment (UE), and wherein the first and second packets are forwarded by a source base station to a target base station during handover of the UE from the source base station to the target base station. 
     
     
       3. The method of  claim 1 , further comprising:
 incrementing a hyper-frame number (HFN) in response to receiving the second packet out of order, the HFN being used to process the second packet. 
 
     
     
       4. The method of  claim 1 , wherein the processing the first packet comprises ciphering the first packet with a first count comprising a hyper-frame number (HFN) and the first sequence number, and wherein the processing the second packet comprises incrementing the HFN in response to receiving the second packet out of order, and ciphering the second packet with a second count comprising the incremented HFN and the second sequence number. 
     
     
       5. The method of  claim 1 , wherein the processing the second packet comprises
 processing the second packet with the third sequence number for transmission to the receiving entity. 
 
     
     
       6. The method of  claim 5 , further comprising:
 receiving a third packet with the third sequence number; 
 re-assigning the third packet with a fourth sequence number later than the third sequence number; and 
 processing the third packet with the fourth sequence number for transmission to the receiving entity. 
 
     
     
       7. The method of  claim 1 , wherein the first and second packets are processed as each packet is received, without buffering the first and second packets. 
     
     
       8. An apparatus for wireless communication, comprising:
 at least one processor configured to receive a first packet with a first sequence number, to process the first packet for transmission to a receiving entity, to receive a second packet with a second sequence number earlier than the first sequence number, the second packet being received out of order with respect to the first packet, to re-assign the second packet with a third sequence number later than the first sequence number, and to process the second packet for transmission to the receiving entity, the second packet being processed as if the second packet is later than the first packet. 
 
     
     
       9. The apparatus of  claim 8 , wherein the at least one processor is configured to cipher the first packet with a first count comprising a hyper-frame number (HFN) and the first sequence number, to increment the HFN in response to receiving the second packet out of order, and to cipher the second packet with a second count comprising the incremented HFN and the second sequence number. 
     
     
       10. The apparatus of  claim 8 , wherein the at least one processor is configured to process the second packet with the third sequence number for transmission to the receiving entity.